This invention is directed to a cathodic electrocoating composition and in particular to a cathodic electrocoating composition containing a morpholine dione crosslinking agent which significantly reduces volatile emissions and bake-off loss that occur from the coating film during curing.
The coating of electrically conductive substrates by an electrodeposition process, also called an electrocoating process, is a well known and important industrial process. Electrodeposition of primers on metal automotive substrates is widely used in the automotive industry. In this process, a conductive article, such as an autobody or an auto part, is immersed in a bath of a coating composition of an aqueous emulsion of film forming polymer and the article acts as an electrode in the electrodeposition process. An electric current is passed between the article and a counter-electrode in electrical contact with the coating composition until a coating of a desired thickness is deposited on the article. In a cathodic electrocoating process, the article to be coated is the cathode and the counter-electrode is the anode.
Film forming resin compositions used in the bath of a typical cathodic electrodeposition process also are well known in the art. These resins typically are made from polyepoxide resins which have been chain extended and then an adduct is formed to include amine groups in the resin. Amine groups typically are introduced through a reaction of the resin with an amine compound. These resins are blended with a crosslinking agent, usually a polyisocyanate, and then neutralized with an acid to form a water emulsion which is usually referred to as a principal emulsion.
The principal emulsion is combined with a pigment paste, coalescent solvents, water, and other additives such as a catalyst to form the electrocoating bath. The electrocoating bath is placed in an insulated tank containing the anode. The article to be coated is the cathode and is passed through the tank containing the electrodeposition bath. The thickness of the coating that is deposited on the article being electrocoated is a function of the bath characteristics, the electrical operating characteristics of the tank, the immersion time, and the like.
The resulting coated article is removed from the bath and is rinsed with deionized water. The coating on the article is cured typically in an oven at sufficient temperature to form a crosslinked finish on the article. The presence of the catalyst enhances the crosslinking of the finish. Cathodic electrocoating compositions, resin compositions, coating baths and cathodic electrodeposition processes are disclosed in Jarabek, et al. U.S. Pat. No. 3,922,253 issued Nov. 25, 1975; Wismer, et al. U.S. Pat. No. 4,419,467 issued Dec. 6, 1983; Belanger U.S. Pat. No. 4,137,140 issued Jan. 30, 1979 and Wismer, et al. U.S. Pat. No. 4,468,307 issued Aug. 25, 1984.
One disadvantage associated with conventional electrocoating compositions containing polyisocyanate crosslinking agents is that in order to prevent premature gelation of the electrocoating composition, the highly reactive isocyanate groups on the curing agent must be blocked, for example, with an alcohol. Blocked polyisocyanates, however, require high temperatures to unblock and begin the curing reaction. This curing mechanism also releases a substantial amount of volatile blocking agents during curing, which generates unwanted film weight losses, also known as bake-off loss, and makes it necessary to purify the exhaust air discharged from the oven and constitutes an unwanted loss in resin solids. In addition, the volatile blocking agents released during cure can cause other deleterious effects on various coating properties, e.g., producing a rough film surface.
U.S. Pat. No. 4,615,779 to McCollum, et al. issued Oct. 7, 1986 suggests the use of lower molecular weight alcohol blocking agents to reduce weight loss when the film is heated to cure. Such blocking agents, however, can produce undesirable film defects. U.S. Pat. No. 5,431,791 to December, et al. issued Jul. 11, 1995 describes the use of a curing agent having a plurality of cyclic carbonate groups, in place of blocked polyisocyanates, which still provides desirable urethane crosslinks but is able to avoid bake-off losses and other problems that accompany the use of blocked polyisocyanate curing agents. Cyclic carbonates, however, are oftentimes difficult to incorporate into the principal emulsion.
Therefore, there is still a need to find new cross-linking agents for cathodic electrocoating compositions that reduce volatile emissions and bake-off losses, while maintaining the desired coating properties.